Track-on-data technique and associated system involving di-bit recording and associated di-gap transducers

ABSTRACT

Techniques and associated apparatus for positioning a magnetic transducer system relative to a selected recording track on a magnetic recording disk. 
     Record information (computer data bits) is recorded along one or more information zones (tracks) of the disk and head-positioning information is derived from this according to a novel &#34;track-on-data&#34; scheme. This scheme uses a &#34;di-gap&#34; transducer system comprising at least one related pair of transducer gaps aligned to register and sense information along a common record track, with the gaps being skewed oblique to the track direction, and aligned transverse to one another. 
     &#34;Servo-bits&#34; (providing head positioning information) and &#34;work-bits&#34; may both be recorded along a given track, these bits, like the mentioned gaps, being skewed relative to the track direction and aligned transverse to one another; the servo-bits in any one track thus being aligned parallel to one of the gaps while the companion work-bits are parallel to the other gap -- these gaps however being functionally interchangeable, with neither gap being dedicated to servo data or work data. 
     Thus, once the di-gap head is moved along a given track, one gap will be aligned with the servo-bits and the other with the work-bits -- the two gaps being adapted to generate respective servo signals and data signals, the servo signal values (phase) automatically indicating the degree of head registration. Preferably, a related data encoding scheme establishes, for each track, the orientation of work-bits and servo-bits.

BACKGROUND OF THE INVENTION

The subject invention relates to magnetic recording systems andparticularly to "head-servo subsystems" therein, i.e., systems adaptedto position transducers radially across a recording disk and thusregister it with a selected track thereon.

Workers in the magnetic recording arts are aware that there is a needfor improved techniques and associated apparatus for properlyregistering transducer means relative to any selected data track on amagnetic record. Efforts have continued for sometime now to fill thisneed.

One such recording system involves a magnetic disk typically used asperipheral memory equipment in a computer system to provide (temporaryor permanent) information storage during computer operations. In onewell-known configuration, one or several such disks are mounted to berapidly rotated in operative relation with transducer means, each diskhaving a multi-track magnetic recording surface on at least one of itsfaces. One, or several, recording heads are, in turn, adapted to bepositioned along each disk-face--i.e., to register with any selected oneof the concentric recording tracks on the disk to record and detect datasignals along any track.

Workers will recognize that optimal use of such media requires thatinformation be recorded at the highest possible density. Present-daytechniques record at densities of up to thousands--in certain cases tensof thousands--of bits per inch. Similarly, workers strive to maximizethe number of circumferential tracks on a disk, with each track asnarrow as possible. Accordingly, with ever-higher bit densities andtrack densities, it is apparent that head-positioning systems are beingpushed to their limit. Systems for quickly and accurately registeringheads with a selected data track are becoming more and moresophisticated, and more complex and expensive, and their operatingparameters more stringent. The invention responds to this need, teachinga novel technique whereby "track-on-data" is feasible.

Workers will recognize that for the typical magnetic disk system arecording head is translated radially across the disk so that themagnetic transducer gaps mounted therein may be selectively positionedadjacent a selected recording track. In this way only a few transducergaps need be used for recording and reading data on a number of disktracks--but to practically implement such a system, a very careful,accurate control of head location relative to the tracks must bekept--and this typically must be done very quickly to minimize accesstime for the computer system served.

For instance, with disks used in a random access magnetic memory thedata bits are recorded in concentric circular tracks so there is acontinual need to secure and maintain very accurate registry of amagnetic transducer with a selected track. Unfortunately, the precisionof the transducer-positioning system will determine track spacingtolerances and accordingly will influence data storage efficiency (bitcompression) significantly--that is the number of characters per unitmemory area will depend upon the accuracy of transducer positioning.Workers have attempted in various ways to improve the accuracy oftransducer positioning, for "servoing" the transducer onto disk tracks.Such systems have commonly employed "position signal" tracks (or tracksectors) interspersed with the data tracks and have also required aspecial servo transducer detector to detect the "position-signals". Theyalso add the operation of writing the servo data. Such featuresinherently degrade data storage efficiency--because of the separateservo transducer system required (e.g., buildup of mechanical tolerancesin the different transducers used; because of the considerable loss ofuseful data recording area to recording position-bits, and so forth).

As workers know (also see U.S. Pat. Nos. 3,691,5431; 3,812,533; and3,838,457) "fine" positioning is typically achieved by controlling themovement of a head positioning carriage in response to the detection ofpre-recorded encoded servo data, using either the data transducer or aspecial servo transducer. The servo data is either recorded on the samedisk as the work data or else on a separate disk, or on a like surfacehaving a precise mechanical relationship with the work data surface.

"Coarse" positioning is typically achieved in either of two ways: (1) bycontrolling radial head movement based on detection of the movement ofthe head positioning carriage, such as by photo-electric detection means(e.g., see U.S. Pat. No. 3,812,533), or (2) by controlling radial headmovement based on detected track crossings, while using recorded servodata for fine positioning purposes (e.g., see U.S. Pat. Nos. 3,691,543,and 3,838,457).

Workers are also aware of various positioning techniques (usingmechanical, hydraulic and/or electro-magnetic means) for registeringmagnetic transducers with associated recording tracks. Certainnoteworthy mechanical or optical techniques are known for monitoringtransducer position relative to a track and providing feedback signalswhich may be used to control a servo positioning-control system adaptedto keep the transducer gap(s) registered with the track. However, noneof the present known techniques for monitoring and controlling headposition is wholly satisfactory--partly because of the extensivecomplex, auxiliary equipment they require and/or because of thelimitation these approaches place on track density. An object of thisinvention is to provide an answer to this problem by teaching improved"track-on-data" techniques and associated systems.

Workers in these arts will recognize that it is quite desirable to"track-on-data", that is to somehow use the area devoted to "data-bits"(i.e., "information signals" developed from certain magnetictransitions) to also provide position control signals which may be fedto a positioning servo and control the positioning and/or alignment of atransducer relative to a recorded track. Obviously, such a technique caneliminate the need for a separate "servo" recording unit and relatedseparate recording zones for servo data (such as separate servo disks orseparate servo tracks, or track-sectors, typically seen in conventionalmagnetic recording systems) since the data-transducer and thedata-recording zones may be used for servo-bits too. The inventionaccomplishes this, providing a "track-on data" system with no need forseparate servo tracks and providing a "servo transducer" which isintegrated with a "data transducer", with servo-bits being incorporatedinto the data recording zones as desired.

Workers will recognize the significant advantages from such a"track-on-data" technique. For instance, present-day magnetic diskmemory systems typically allocate servo-bits to special "servo tracks"(either on a special portion of each disk or on a special disk in eachfile) dedicated to this purpose. Workers will also acknowledge thatpresent-day systems commonly detect transducer positioning (servo)signals according to amplitude-modulation techniques (i.e., byvariations in the amplitude of position-indicating recorded magnetictransitions, or "servo-bits"), and that this is less than optimal. Forinstance, the amplitude-sensitive transducers typically required are alltoo subject to "noise". Since erroneous amplitude variations can resultfrom many common sources, such "noise" makes the servo systems based onthis approach subject to serious error. An example of this approach isfound in U.S. Pat. No. 3,864,740 to Sordello, et al. and in U.S. Pat.No. 3,185,972 to Sippel and in U.S. Pat. No. 3,614,756 to McIntoch, etal.

The Sordello patent in particular will indicate the lengths to whichworkers have gone to try to compensate for the difficulties arising fromamplitude sensing. That is, Sordello will be seen to represent a "trackfollowing" method of detecting transducer position wherein prerecordedtracks are positioned on a recording medium so as to facilitate thedirect detection of transducer position relative to the medium. Arelated U.S. Pat. No. 3,404,392 to Sordello discloses a track-followingservo system. In this system a special pair of servo tracks is laid downon either side of each data track, the servo-bits in one of these servotracks being recorded at one signal frequency and those of the other ata second frequency, the corresponding servo-output signals beingfrequency-separated by electronic filtering means which generated asummed servo signal. With such a system it is imperative that thefrequencies of the two servo tracks differ sufficient to permiteffective filtering and signal separation since the associated detectingtransducer was "reading" both tracks simultaneously.

Conversely the first-named Sordello U.S. Pat. No. 3,864,740 postulates apair of adjacent servo tracks wherein equal-amplitude servo signals areimpressed, the servo tracks being prerecorded on the medium with arelatively inconsequential frequency difference; then, upon detection,the resultant servo signals are frequency-multiplied. That is, a pair ofservo signal detection means are provided to modulate (multiply) thetransducer output with modulating signals at the frequency of the firstand second servo track waveforms and thereby generate a summedservo-output. This output represented the frequency difference betweenthe original servo signal and respective modulating signal. By detectingthe magnitude of this output (difference) signal, servo signals aregenerated for regulating servo positioning means.

Such a system will servo the transducer into registry with a selecteddata track. Workers will realize that the servo tracks flanking eachdata track represent a single continuous linear recording at twodifferent encoded frequencies. Thus, if a single transducer is arrangedto simultaneously read a data track and the flanking servo tracks--alltogether--and if means are provided to filter the servo information fromthe data signals, and then compare the two servo signals: then, one maydevelop a "position-error" signal and apply it to an actuator-servo unitto reposition the transducer.

However, such a system has the inherent disadvantage that the data andservo-bits must be separately recorded and at widely-spaced frequencies.Also, the servo frequencies cannot be harmonic of one another lest therebe any harmful interaction between the (data and the servo) outputs.Another serious disadvantage, is that such a magnetic transducer willhave a different transfer function for the data bits (frequency) then ithas for the servo-bits (frequencies); and this can introduce furthererror.

Similarly, in the cited McIntoch patent a transducer positioning systemis taught which comprises a magnetic disk with servo tracks and datatracks, with the magnetic domains of the servo tracks orientedrelatively orthogonal to those of the data tracks. A transducer isprovided to generate two outputs--a "data output" representing the rateof intensity change of the magnetic data domain and a "servo output"representing a function of the absolute magnitude of magnetic fieldrepresented by the magnetic servo domains. A flux-sensing portion of thetransducer detected this servo output and thus indicated the transducerposition relative to the data track, presenting an "error signal" to aservo positioning means.

One feature of such a servo system is that it provides a headrepositioning-(or servo-error-) signal which is independent of mediummovement relative to the transducer--that is, the acceleration ordeceleration of the medium will not effect transducerresponse--evidently because the flux-sensing means will provide theprescribed output independent of whether the medium is moving atdifferent speeds or is motionless. Also such a servo system provides fororthogonal isolation between (the magnetic influence of adjacent)servo-bits and data bits so recorded. This invention provides the sameadvantages while eliminating the need for a separate servo track. Otherapproaches are known which involve separate servo tracks (e.g., U.S.Pat. No. 4,007,493 to Behr, et al.; U.S. Pat. No. 3,964,094 to Hart);where, by contrast, systems according to the invention do not.

Workers are aware of present-day magnetic recording systems that useprerecorded servo tracks (e.g., see U.S. Pat. No. 3,903,545 to Beecroft,et al.; No. 2,938,962 to Konins, et al.; No. 3,404,392, to Sordello; andNo. 3,185,972 to Sippel). One implementation involves a stackedmulti-gap transducer adapted to register an intermediate head-gap over a"selected" data track while using a pair of flanking gaps to readservo-bits from a pair of servo tracks flanking each data track.

Invention features:

The present invention is a significant improvement over such techniques,teaching the use of a di-gap transducer array, with a gap pair orientedto be orthogonal to one another as well as disposed "in-line", alongtrack-direction. These gaps are adapted to conjunctively read twodifferent kinds of (data/servo) bit sets arranged along the track, onekind aligned with one such gap, the other kind at right angles and thusaligned with the other gap. The servo-bit locations may indicatehead-misregistration and, as detected, do so in terms of elapsed timebetween prescribed servo signals along any given track--as opposed tocomplex, fussy frequency modulation systems or amplitude modulationsystems.

According to one feature, systems according to the invention so functionwithout inter-track "guard-bands" or the like; disposing adjacent datatracks into abutment with one another. Also, the magnetic datatransitions are skewed in parallel along one track while beingskewed--orthogonal to this track--along both adjacent tracks. Asexplained, this can minimize adjacent channel interference and alsodispense with the need for such things as "guard-bands". According toone embodiment, servo positioning signals are interspersed amongdata-bits and detected with a single "di-gap" transducer head. ["Di-gaphead" hereinafter referring to a pair of positionally-related magnetictransducer heads, each head comprising a pair of pole pieces separatedby a transducing gap and wound with an associated coil-activationcircuit--through the windings and one pole-piece may, of course, beshared]. Workers will recognize the advantages of this approach; forinstance, eliminating the need for separate servo heads and recordingoperations--as opposed to the prerecorded ("initialized") disks incommon use today.

Of course, others have contemplated the use of "orthogonal data tracks"(e.g., see the cited Sippel patent). Likewise, others have thought aboutmonitoring head registration according to the alignment of a transducergap relative to an array of parallel magnetic domains arrangeddiagonally across a recording track (e.g., see the "herringbone" servotracks and related detection technique taught in U.S. Pat. No. 3,686,649to Behr).

However, the instant "track-on-data" arrangement will be distinguishedas novel and unexpectedly effective, combining the "adjacentorthogonal", abutting track arrangement--with data bits arranged inoblique "herringbone" pattern and "servo-bits" interspersed andorthogonal--with a related di-gap servo/data transducer array--thedi-bit pattern being sensed by the associated "di-gap" head--e.g.,preferably with one ("servo") gap thereof being adapted to detect ashift in lateral head position (misregistration) according to a varyingtime interval between detected servo signals. (E.g., between a referenceservo signal and a variable servo signal). Of course, workers haveheretofore suggested abutting orthogonal skewed data tracks along amagnetic record as well as superposition of recording transitions.(e.g., U.S. Pat. No. 2,929,670 to Garrity). However, it will be seen asnovel to so use di-bit recording with di-gap heads, allowing eitherservo data or work data, or both, to be transduced by either or bothgaps.

In a preferred embodiment for instance, the work-bits are impressed"skewed", at a prescribed angle oblique to track direction, whileservo-bits are arranged along the same track with their magnetic domainsaligned transverse to these "work domains" and disposed at prescribedregular intervals along the track. Such work-bits and servo-bits areoriented to interact with a common double-gap transducer unit, onemagnetic gap aligning parallel to the work-bits, the other aligningparallel with the servo-bits,--thereby maximizing the respective dataoutput and servo output signals. Such a system obviously maintains afixed spatial relation between data transducer and servo transducer veryconveniently, as well as keeping them inherently synchronized (that is,they traverse the medium at the same speed and direction).

According to a preferred embodiment this arrangement is capable ofeasily providing "registration feedback". That is, with this techniqueand associated apparatus, the multi-gap transducer head may beregistration-referenced to the contemplated magnetic recording medium(moving along a prescribed direction), and may be repositioned forcentering therealong.

At least one gap pair is used, with the paired work gap and servo-gapboth skewed vs. the track axis and transverse to one another. The servooutput is coupled to a head-positioning arrangement adapted toreposition the gap pair radially on a disk for centering over a selectedtrack. Preferably, the servo-bits, (head positioning information) aswell as "work-bits" are both recorded along the same track, so that thework-bits pass in alignment with one of the transducer gaps, while theservo-bits align parallel to another gap.

Thus, once the di-gap head is registered on a given track its gapsshould register with the servo-bits and the work-bits respectively so asto generate respective servo signals and data signals. According to thisfeature, either gap may handle either signal.

Preferably, the gap sensing servo-bits will provide a servo outputreflecting the time interval between successive servo-bits--this, inturn, reflecting any shift in head position to the left or right of thetrack center line. Preferably, the servo signals "follow" the transittime of the head; accordingly, the servo output may be interpreted as adistance-indicating signal whose variance from a prescribed norm(representing perfectly registered, or centered, head over a subjecttrack) represents the lateral head variance or misregistration and thusmay be used to cause a responsive servo system to reposition and centerthe head (known systems which seek a "zero error" feedback signal). Sucha servo output control over the radial positioning of the head can beimplemented, using conventional servo means, as known in the art.

The servo output may be compared with a reference signal representing"registration" (centered alignment of the head along the track--e.g.,via "Table-Lookup") to derive a servo controlling "difference" (error)signal controlling the servo to correct by repositioning the transducerleftward or rightward to achieve "zero-error" (i.e., registration).

Preferably, such an arrangement is used with disks having "adjacentabutting" data tracks with data bits aligned oblique and parallel alonga track and orthogonally between tracks, with a respective pair of dataand position gaps disposed "in line" to be translated along a selectedtrack to develop data and servo signals both therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated by workers as they become better understood by reference tothe following detailed description of the present preferred embodimentswhich should be considered in conjunction with the accompanyingdrawings, wherein like reference symbols denote like elements:

FIG. 1, including 1A, is a very schematic plan view of a magnetic diskrecord with a simple record track indicated as position encodedaccording to one embodiment; FIG. 1A showing an enlarged area of thedisk as disposed relative to a di-gap transducer provided according tothe invention;

FIG. 2, including 2A-2C, indicates a single record track, encoded byposition and disposed relative to associated transducer indiciaaccording to a related embodiment; while FIGS. 2A, 2B and 2C indicatesuggestive transducer dual-outputs for three illustrative transducerpositions;

FIG. 3, including 3A and 3B, illustrates a fragmentary plan view of aportion of another magnetic disk record, greatly enlarged, with adjacenttracks and magnetic domains, including another position indiciaembodiment therein schematically indicated in positional relation withan associated "di-gap" magnetic transducer; an enlarged showing of oneof these tracks and the associated transducer being indicated in FIG. 3Aand related indication of illustrative transducer outputs in FIG. 3B;

FIG. 4 illustrates, in plan view and greatly enlarged, a schematizedfragmentary showing of a set of tracks like those in FIG. 3 as relatedto "double di-gap" transducer means and data bits indicated according toanother embodiment;

FIG. 5 is a schematic block diagram of a utilization system adapted for"di-bit/di-gap" embodiments like the above;

FIG. 6 is similar to FIG. 2, indicating a modified form ofposition-encoded indicia and associated transducer gaps along a track;and

FIG. 7 is a plan view of a disk record as in FIG. 1, schematicallyindicating a related mode of recording position bits.

DESCRIPTION OF THE PREFERRED EMBODIMENT

General description, background:

FIG. 1 schematically illustrates a magnetic recording disk D constructedaccording to principles of this invention. This, and other magneticrecording means discussed herein, will generally be understood asconstructed and operating as presently known in the art, except whereotherwise specified. Such a recording disk is relatively conventional,being about one foot in diameter and formed of a non-magnetic substratematerial with a thin magnetic recording medium applied to one face (orboth faces).

Such disks are commonly mounted on a turntable which is seated on aprecision-bearing spindle and rotated at high speed during operation.Thus, every bit along any selected circumferential track (seeillustrative recording tracks T-r, T-1, T-2 and T-3 in FIG. 1 and 1A) isswept past magnetic transducer means (here, see transducer head "h" onarm TT) many times a second for minimum access time and high speedread-out. In a commercial disk unit, one or several such disks isemployed, each having one or several transducers per disk.Conventionally, record information ("work-bits") is recorded along thedata tracks. One or several servo-transducer means is often provided andarranged to be operated in conjunction with special servo-tracks (ortrack-sectors) on which servo-data (servo bits) is recorded.

The rotational rate (rpm) of disk D is normally carefully controlled sothat any small relative change in rotational speed is quickly correctedand so the magnetic reading and writing operations are kept constant.Conventionally, the rotational position of the disk is indicated byangular encoding means, clock tracks, index indicia (e.g., hole i-halong "start radius, SR", FIG. 1), or the like. Such features areconventional and not detailed here since they are not of specialsignificance in practicing the invention.

This servo information is used to align a transducer head (here, seehead "h" mounted on transducer-translator unit TT and conventionallypositioned thereby) radially of disk D, i.e., with its transducer gapsregistered with a given selected track on disk D, using means well knownin the art (and not shown or described here). A reference radius isconventionally indicated by passage of an index indicium over a relateddetector (e.g., here, see index hole "ih" along "start radius" SR).Complicating this servo function are the typical deviations of disktracks from perfect concentricity (with one another and/or with theirdisk). This is indicated by tracks t-i, t-ii which will be understood asout of concentricity, both with disk D and with reference track T-2.

Representative tracks of disk D are fragmentarily shown--exploded andwith recording bits also illustrated--in FIG. 1A along with an enlarged,very schematic showing of head "h", understood as adapted for recordingand detection of such bits.

As is conventional, it will be understood that work bits in the form ofmagnetic signals "wb" are recorded on the face of disk D by atransducer-gap provided in the transducer array (here, represented byeither of the gaps "g" of "di-gap head" "h") which is appropriatelypositioned. Head "h" will be understood as coupled mechanically to apositioning unit (not shown) including position-actuator means arrangedand controlled so that transducer "h" is translated radially across D tobe operationally superposed over a selected track (very schematicallyindicated by head translation arm TT and generally understood in theart, hence no details are indicated or necessary).

Thus, for example, the position control can comprise a so-called "voicecoil actuator", i.e., a magnetic positioning device very similar to thatemployed in loud speakers. Such actuators are generally preferredbecause they are inexpensive, fast, accurate and readily controlled withconventional circuitry. However, if desired, other mechanical,electromechanical or hydraulic positioning control means may beemployed, as is well known in the art.

Work-bits "wb" will be understood as recorded along the disk tracks byappropriate energization of the related magnetic transducer gap in head"h", and to comprise a sequence of magnetic transition signals ofidentical encoded magnetic polarity and alignment. Here, preferably, andaccording to a feature aptly employed with the invention, work-bits "wb"are impressed at a common prescribed angle to be all similarly obliquewith respect to the respective track axis A_(t). (The direction oftranslation of the track with respect to the transducers).

According to one feature, a bit encoding scheme establishes, for eachtrack, a prescribed alignment for work-bits and for servo-bits, thisalignment being arranged to automatically indicate registration, orcentering, of transducers with respect to a track). For instance, fortracks T-1, T-2, T-3, etc., in FIGS. 1, 1A, it will be understood thatthe work bits "wb" are aligned in one oblique direction along all"odd-number" tracks (e.g., T-1, T-3) and aligned transversely in all"even-number" tracks (e.g., T-2). The servo-bits "sm" are alignedorthogonal to the work-bits along any given track. Bits sm will becharacterized as "centering dibits" as explained below and will beunderstood as configured like "V", or Greek lambda "Λ" relative to thetrack. (e.g., in contradistinction with "tilted V" marks such as shownin U.S. Pat. No. 4,074,328 to Hardwick).

These recording tracks will be understood as schematically representedhere by a mere sector of the full disk/track circumference; there beinga great number of such tracks understood as concentrically arranged, inabutment, on disk D. Each of the concentric recording tracks is distinctin location, length and curvature and the transducer and associated gapsmust be very precisely registered thereon before writing or reading maybe properly performed--as workers in the art very well know.

Thus, in order to position a transducer gap in operative (read/write)relation above a selected track, the position of the transducer headmust be detected and checked for "centering" along this track.Conventionally, a raw position signal would be compared with a referencesignal which represents the "centered" (or registered) head position.Any discrepancy between these indicates the degree of misregistrationand is processed to result in an "error signal" fed back to a positioncontroller (in a conventional servo loop) for precise repositioning andcontrol of the head.

In some known systems, track location is indicated by "servo-bit"magnetic transitions provided along special dedicated servo tracks, of adisk. In one well known system, such servo-bits are recorded alongspecial servo tracks disposed between adjacent data tracks, assuggested, for instance, in the cited U.S. Pat. No. 3,686,649 to Behr.By contrast, in the subject embodiments, the data tracks are abutted(radially) with the bits aligned to be orthogonal between adjacenttracks--doing away with intervening spaces for servo tracks,guard-bands, etc.

Such a "radially-abutting" track configuration will be recognized asfacilitating maximum track density. However, this makes it necessary torecord servo information bits along a portion of the data tracks. Italso makes it critical that the read/write gaps be positioned veryaccurately, since a transducer gap might otherwise easily drift over anadjacent recording track. These requirements are met in systemsaccording to the invention, with a "di-gap/di-bit" servo controldescribed hereinafter. That is, a transducer is provided with a pair ofskewed orthogonal gaps, [such as gaps g₁, g₂ of head "h" in FIG. 1A];and related servo bits are recorded to comprise at least one pair ofsimilarly-skewed, mutually-orthogonal servo-bits [such as servo-bits"sm" in FIGS. 1 and 1A].

Fig. 1 embodiment:

Turning to the details of FIGS. 1 and 1A, the indicated magnetictransducing arrangement will be understood as adapted to operate uponthe abutting concentric data tracks T-1, T-2, T-3, etc., of a magneticrecording disk D, when the disk is rotated in the known manner.Servo-bits "sm" in the form of skewed, mutually-orthogonal di-bits willbe understood as distributed in a prescribed manner along each giventrack T, being inserted among the work-bits "wb", along with(track/sector) identifying bits "sb". Work-bits "wb" are aligned inparallel, and similarly skewed, along every track, being "orthogonal"between adjacent tracks (i.e., they are "skewed andadjacent-orthogonal"). This magnetic information will be understood asconventionally recorded and sensed. Such is indicated suggestively by"di-gap" read/write head "h" adapted to be moved radially across disk Dto any selected track-location where the transducer gaps will liesuperposed above a "selected" track T and centered therealong in close"read/write relation" therewith (see FIG. 1A).

According to one recited feature, head "h" includes at least one"di-gap" array; i.e., a pair of skewed orthogonal gaps g₁, g₂. Head "h"has a width w_(h) approximately equal to the (uniform) radial width ofw_(t) of the tracks. Thus, when head "h" is moved into prescribedregistration (exactly-centered relation) with a given track T, one ofits gaps "g" will align and superpose coincidently with one set of bits(--either "wb" or "sb"; or one half of di-bits "sm", these bitsextending across the track in a "first" skew-alignment--) while thecompanion gap will be similarly aligned with a second set of bitsorthogonal to the first. Thus, gap g₁ in FIG. 1A, passing along trackT-4, (or any "even-no.-track") aligns with "track-sector identifying"bits "sb", and with one of the servo di-bits "sm", while the other gapg₂ will align with work-bits "wb", and with the other half of di-bit"sm". Conversely shifting head "h" to be centered along track T-1 (orany "odd-no.-track") will reverse this registration-relation, e.g., g₁,aligning with the work-bits "wb", there.

The "magnetic congruency" of such a di-gap transducer and such servodi-bits, (or "chevrons") when a head is centered over a recording trackwill be seen as facilitating advantageous servo control--with the degreeof off-centering, or misregistration, being sensed according to howclosely such a di-bit "fits" the passing di-gap--as further describedand explained below. This concept will also be better understood uponconsideration of the alternate embodiment in FIG. 2 described asfollows.

Fig. 2 embodiment:

Here, a "di-bit" data track T' is shown in operative relation with anassociated "di-gap" transducer assembly TR' understood as passing overrepresentative track T' in the direction of the arrow, in magneticallyoperative relation with bits A, B, therealong, but in slightmisregistration, with respect thereto (misregistered "to the left").Transducer TR' thus includes a pair of skewed, mutually-orthogonaltransducer gaps g_(a), g_(b) aligned oblique (here, preferably 45°) tothe axis of track T'. Track segment T' may be understood as aservo-sector like these indicated in FIGS. 1, 1A with similar "di-bits",comprised of relatively orthogonal first and second di-bit-halves,(i.e., magnetic recording half-bits A and B respectively, analogous toservo di-bits "sm" above). Di-bits A, B will be understood as eachaligned with a respective transducer gap (g_(a), g_(b)) and thusorthogonal to one another and oblique (also 45°, preferably) to thetrack direction (arrow). Track sector T' shows only servo bits and nowork-bits, and is here represented as "linear" rather than "curvilinear"(as the actual face of a magnetic recording disk would be)--but only tosimplify the explanation. Those skilled in the art will obviously extendthe principles explained to curved disk tracks, and other relevantmedia, including work-bits too, as a matter of course.

Now, it should be understood that the relative disposition of the bi-bitpair A, B and gaps g_(a), g_(b) (their spacing and relative angularorientation) is such as to render "magnetic congruency" between a bitand a respective gap when head TR' is centered exactly (registered)along track T'. Thus gap g_(a) will coincide with bit A and gap g_(b)will coincide with bit B at a certain time during track passage--and thedetecting transducers will thereupon generate a corresponding pair ofcoincident opposed "servo-output" pulses. A relatively perfectcongruence of di-gaps and di-bits is signalled in FIG. 2B by thesimultaneity of symmetrically-opposed pulses P-A, P-B representingdetection of bits A-1, B-1, etc., by head TR'; pulse amplitude may beequal, but this is not always necessary, though it facilitates a"null-balance" indication of perfect registration, of course, as workerswill recognize). This opposite polarity output is readily rendered byknown methods.

On the other hand, a slight misregistration of head TR' to the left, asdepicted in FIG. 2, should produce the kind of (successive, spaced)non-coincident output pulses indicated in FIG. 2A wherein pulse P-A'(produced by passage of gap g_(a) over bit A-1) will obviously occursomewhat earlier in time than the companion output pulse P-B'--P-B'reflecting the later passage of gap g_(b) over companion bit B-1. And,the time-discrepancy+ΔT_(L)) between output pulses will be a measure ofthe degree of head misregistration (the positive sign indicatingmisregistry to the left), as workers in the art will appreciate.

Conversely, rightward misregistration of transducer TR' is signalled inFIG. 2C (such head misregistry not shown here, but indicatedschematically by "phantom bits" A"-3, B"-3 in FIG. 2) by thenon-coincident servo (timing) pulses P-A", P-B". Here, output pulse P-B"from gap g_(b) "leads" output pulse P-A" from companion gap g_(a) by atime delay (-ΔT_(R)) which corresponds to the degree of rightwardmisregistration (and accordingly is designated as negative delay time).

Workers will visualize other like arrangements for correcting headmisregistration wherein transducer di-gaps align oblique to thetransport direction can likewise interact with associated (similarlyoblique) di-bits so that exact head/track registration is indicated bycoincidence of the paired output pulses, while misregistration isindicated by positive or negative non-coincidence. In certain cases suchservo-bits may, themselves, constitute work-bits ("record data"),especially where the work-bits are skewed 90° between adjacent tracks(as in FIGS. 1 and 2).

This is indicated very schematically in FIG. 4 where segments ofabutting adjacent disk tracks T-a, T-b, T-c, T-d, T-e is depicted ascomprising skewed work-bits "wb" which are "orthogonal adjacent".According to this feature, periodically a high-frequency burst of"special-bits", hfb, hfb'--similarly skewed--are laid down forhead-centering purposes. Workers will appreciate that bits hfb, hfb' maysubstitute for di-bits like "sm" above. It will be evident, that,according to another mode of "di-gap/di-bit" operation a transducer suchas TTR with at least one related di-gap pair (see orthogonal gaps g_(a),g_(b)) may be provided, with the gaps spaced "across tracks" (ratherthan "along a track" as above) by a prescribed (odd) number of trackwidths (here, three--this distance accommodating reasonable gap spacingand head manufacturing tolerances). Di-gap g_(a), g_(b) will sense thehigh-frequency bit pair hfb, hfb' coincidently as an indication thathead TTR is perfectly centered over tracks (T-b, T-e); non-coincidenceindicating misregistry, as before.

It will be evident that the di-bit pair can be variously offset in space(e.g., by offset spacing dd-g, the same as offset d-g between gapsg_(a), g_(b)) or offset in time (electronically)--gap-to-bit coincidencebeing provided for either spatially or electronically. Of course, such"work-bits" may be otherwise modified and distinguished to thus serve as"servo-bits"--for instance, written as "invalid bits" (e.g., preceded bythree or more (3+) "zeros") or as "special bits".

Also, the head TTR may include a second orthogonal gap pair (here,g_(c), g_(d)) to accommodate the sensing of work-bits in other tracks(e.g., T-a, T-c, T-e for g_(c) ; T-b, T-d, etc., for g_(d)). Thesesecond gaps are not needed however and will typically be included onlywhere cost-justified.

Similarly, one oblique gap may be translated along such a track todetect certain aligned servo bits and to generate servo-positioningpulses, with these pulse signals "referenced to center" according totheir degree of coincidence with a prerecorded set of "clock signals".The servo output may be time-referenced to such clock signals(indicating "center-track registration" of the servo gap) and thuseliminate the need for contemporaneous detection of a second servo-bitwith a second orthogonal gap, as described above. In certain cases(e.g., FIG. 1A) part of a track-sector will include only servo-bits,with abutting "blank track segments" flanking the sector radially, toeliminate risk of noise. This is preferred for this "track-on-data"operation and maximizes data compression.

In any event, the foregoing techniques will be appreciated asfacilitating a true "track-on-data", or "servo-on-data" operation,locating the servo-bits directly adjacent their associated data bits andavoiding the difficulties and errors of remotely-located servo indicia.

Operation:

Returning now to FIGS. 1 and 1A, it will be apparent that a simple,convenient mode of operation may be used to develop "centeringinformation" from servo di-bits "sm" by proper positioning andmanipulation of the associated set of skewed di-gaps g₁, g₂ of head "h".Generally speaking, this will involve head "h" being coarsely-positionedover a given "selected" track; then the servo di-bits being sensed fortheir degree of congruence with di-gaps g₁, g₂, so that any minorcentering-adjustments necessary can then be made--then, withregistration thus assured, optimal data transfer (read-out or writing)may proceed.

More particularly, an illustrative, step-by-step servo sequence is nowdescribe as follows (see FIG. 1):

Assumptions:

The control memory will contain a formatting code indicating the"normal" position (i.e., associated delay-time post "start time") ofservo-bits "sm" on each disk track when the head is "centered" there. AsFIG. 1 indicates, servo-bits "sm" are distributed about each track T[preferably at the "preamble" to each "data sector", being followedthere by "identification bits" "sb", then by the record data (work-bits"wb")] using a frequency matched to servo response. That is, for a givenmedium velocity, the servo system will have a certain nominal responsetime for carrying out head-centering position adjustments. For instance,it may be able to complete a repositioning sequence within about 1/10 ofa disk revolution--accordingly bits "sm" should be disposed every 36°,or less, for optimal servo response.

Bits "sm" may be prerecorded when a disk is "initialized". Also theformat for orientation of work-bits "wb" will be predesignated so that,for instance, bits "wb" will be aligned with gap g₁ for all the "oddnumber" tracks (T-1, T-3, etc.), and aligned with gap g₂ for all the"even number" tracks (T-2, T-4, etc.).

And preferably, the servo-bit preamble to each sector, such as indicatedat di-bit "sm" in FIG. 1A, will be arranged so that the adjacentabutting track segments are "blank", with no bits recorded thereon, thusavoiding interfering pickup if head "h" is misregistered along asector-preamble.

The "servo preambles" for alternate tracks may be aligned along a commonradial-direction as in FIG. 1A, or else be offset rotationally so as notto interfere with this operation.

First step: Head "h" enters the track area and is roughly centered bycoarse positioning means (known, not described) over "selected" trackT-4. Now a "centering sequence" is invoked before any data transfer(i.e., "read-out" or "write-in") is attempted;

Second step: At an appropriate "start time" (e.g., following the passageof index hole "ih", as is well known in the art), the servo controlstage will begin to monitor the output from gaps g₁, g₂ to "look for"di-bit sensing and related "servo output" for both transducers. That is,the next following prerecorded servo di-bit or "chevron", "sm" willinitiate this servo output (as in FIGS. 2A, 2B, 2C) and be used toidentify the track, data block, etc., as well as in a "centering check".

Third step: Centering of head "h" takes place responsive to the outputsensed from chevron "sm"-4 (functionally, along the lines indicatedelsewhere). Thus, if one servo output pulse is significantly (e.g.,beyond a given range) out of coincidence with the other servo outputpulse, head "h" will be shifted to correct this--then, with the gapsproperly centered, the centering-check is repeated, data blockidentified, etc., and--finally--read-out of work-bits may begin.

Fourth step: With the gaps now properly "registered" with the bits along"selected" track T-4 (FIG. 1A) optimal, accurate data transfer maybegin. Accordingly, the gap g₁ output might then be gated to read-outthe track number and sector number indicated by (aligned) sector bits"sb"; and thereafter to read-out the work-bits "wb" sensed by companiongap g₂.

Workers will appreciate how simple such a servo centering technique isand how it assures that the head is centered over a track before anyread-out may begin. This maximizes the gap/bit congruency and theassociated accuracy and efficiency of bit detection and/or writing,while at the same time accommodating maximum track density with lessconcern for head misalignment.

More particularly, a known digital servo system may be set to invoke acertain number of "positioning steps" when translating head "h" fullyacross a track width (e.g., 30 steps to cross a bit-track completely).And a "centered" limit may be set such that a certain minormisregistration is tolerated (e.g., up to 3 servo steps, or the "timedisparity" T_(L) between di-gap output pulses which corresponds to this,as indicated in FIG. 2 above).

Workers will recognize that servo response to the misregistration (ofthe di-gaps with the servo di-bits) detected may be a relatively gradualand continual thing. That is, with many sectors and associatedsector-preambles normally occurring along a given track, the servopositioning means may be referenced to "coincident di-gap output"several times per disk revolution, with head "h" being responsively"recentered", to some extent, each time a servo di-bit passes. Thisfacilitates a smoother, more gradual and continuous, "centering" mode asworkers will appreciate. It also helps to compensate for diskeccentricity and "run-out" along any given track--something quitecomplex and difficult to accommodate with conventional means.

Eccentric tracks:

Workers will recognize various applications of the above features; forinstance, when tracks are recorded in parallel about a disk to be out of"concentricity" with the disk somewhat. Such "eccentric tracks" arefamiliar to workers and occur not infrequently (e.g., due to localvariations in turntable/shift alignment, etc.). Such are illustrated byeccentric tracks T-i, T-ii, in FIG. 1.

A solution to this problem, along the lines taught, is suggested in FIG.7 where a pair of di-gap heads h₁, h₂ are mounted in tandem tosimultaneously operate on two respective tracks on a disk DD--one trackT-1 being one of several carefully centered "prerecorded servo tracks"(or sectors); the other being a new data track (e.g., T-a), whoseconcentricity is to be assured. Accordingly, it will be evident thathead h₂ may be "slaved" to head h₁ and be servo-centered therewith, sothat when head h₁ is registered on a "control servo track" (or tracksegment T-1, using di-gap/di-bit sensing as taught above), head h₂ will"follow" and write a related data track which is concentric with T-1 andwith disk DD (e.g., T-a "slaved" to T-1, T-b slaved to T-2, etc.).Further, a "bootstrap mode" may be used whereby certain other new datatracks are, in turn, likewise "slaved", during writing, to (the positionof) a respective "first-written" data track (e.g., T-n and head h₂slaved to T-1 and head h₁).

Min/max misregistration:

A maximum tolerable misregistration may be built-in to the system bylimiting the minimum acceptable amplitude of the servo di-bit read-out.For instance, a 50% misregistration (or 15 "servo-steps" misregistry asabove) might be the maximum centering error tolerable; in such case thegaps would be "straddling" two adjacent tracks and the nominal amplitudeof the servo read-out pulse corresponding to this might be about 50% ofthe maximum "fully-centered" read-out amplitude.

In such a case, (and in any instance where a head is so "straddling")the servo control program might be set to arbitrarily command the headto take predetermined corrective action (e.g., "move radially-out tensteps and begin a new centering sequence"). In a related situation, ifsatisfactory centering were achieved over the wrong track; then uponreading the track number (sensing bits "sb") this would be discoveredand no harm done (e.g., no erroneous data read-out would beaccepted)--instead, an entirely new "coarse positioning" step could beinvoked to re-new the search sequence for the correct selected track.Workers in the art will conceive other conventional ways of handlingsimilar problems using the invention.

Workers will also conceive other ways of employing such servo di-bits.For instance, as described below relative to FIGS. 3 and 3A, one or bothoblique portions of a servo di-bit may be interspersed with, or"over-written" upon, work-bits.

Embodiment of FIG. 3:

A modified, further improved way of using a skewed orthogonal di-gaptransducer, together with similarly-skewed servo-bits is indicated inthe embodiment of FIGS. 3 and 3A. Here, a rotatable magnetic disk recordD is shown very schematically, with abutting recording tracks (e.g.,T-1, T-2, T-3) along which indicated "work-bits" "d" may be recorded asabove. Bits "d" will be understood as aligned in parallel oblique to thetransport direction (arrow T-A_(X)) and orthogonal between adjacenttracks. A series of "singular" skewed servo-bits "smm" (not di-bits) arehere "written-over", or interspersed among, certain work-bits "d" andorthogonal thereto according to this feature. A di-gap head TR may beused for transducing as before.

Workers will appreciate such a "track-on-data" scheme--with a di-gaphead operating on "di-bit tracks" (here, the servo-bits and work-bitscooperate--not pairs of servo-bits) as quite new and advantageous--andone that may be implemented in various ways. The magnetizationsrepresented by these bits and their respective magnetic direction are(as above), indicated schematically by the parallel lines "d", "smm".The "orthogonality" between bits may be visualized as a 90° rotation ofthe magnetic axis of the pre-magnetized domains on the virgin substrate.

Except as otherwise described disk D and associated di-gap head TR (withskewed orthogonal gaps g, g' and accessories, like access arm AA, etc.)will be understood as conventional and similar to those described abovewith respect to FIG. 1. Thus, the orthogonal skewed transducer di-gapsg, g' are disposed and adapted to be aligned in "magnetic congruence",or registration, with either of bits "d" or bits "smm" (head TR having awidth w_(h) approximately equal to the width w_(t) of the tracks T).Thus, bits "smm" indicated in phantom in FIGS. 3, 3A will be understoodas impressed in a prescribed manner over a track sector corresponding toa prescribed plurality of transverse work-bits "d" (--rather than beingallocated to separate tracks and/or to separate portions of a datatrack). Here, servo-bits "smm" span four (4) work-bit sites, forexample.

As mentioned above, bits "d", "smm" are skewed, i.e., with respect to"translation axis" T-A_(X), at angles φ and φ' respectively (thesepreferably being +45° and -45° with respect to TA and thus transverse toone another) and are at right angles with like respective bits from"track-to-track". Such an "adjacent orthogonal/transverse within track"bit array has been described above. Accordingly, as head TR movesradially across disk D, it will be understood as traversing successivedisk tracks to stop "semi-registered" over a selected track; then thebits "d", "smm" will be detected by the passing gap that happens to bealigned therewith and "fine-registration" will follow.

Thus, for instance, when head TR is positioned so that gap "g" overliestrack T-1 it will be relatively perfectly aligned to record and/or readwork-bits "d" with maximum efficiency (being parallel thereto and"magnetically congruent" when superposed)--while companion gap g',aligned transverse to gap "g", will accordingly be oriented for nosignificant magnetic interaction with bits "d", and for maximalinteraction with bits "smm" (gap "g", in turn, being relativelyunresponsive to bits "smm").

Likewise, shifting of head TR to overlie track T-2 will reverse theoperative sense of gaps g, g' so that gap g' is uniquely operative withdata bits "d", (which are orthogonal to the work-bits "d" along trackT-1), while gap "g" lies in exclusive operative alignment withservo-bits "smm". Shifting again to register TR along track T-3 willagain reverse the operative senses of the gaps (so they are the same asfor T-1).

Workers will understand that the recording and reading is performedconventionally, with narrow magnetic gaps g, g' traversing domains inswitching-relation so that if a domain ("bit") is aligned relativelyparallel to a given gap and translated there-past in "magneticcongruence", a full-scale read-out signal will be induced from theassociated transducer reflecting this passage. The polarity of theinduced signal will depend, of course, upon the direction of change ofmagnetic transition.

According to a feature hereof, the distance interval between a pair ofadjacent servo-bits along a given track (see interval S between bits"smm" along track T-1 in FIG. 3) will correspond to a prescribed"reference interval", reflecting a prescribed controlled head/diskvelocity. When a perfectly registered (or centered) transducer proceedsalong the subject track at the prescribed constant speed, servo outputpulses "sop" will issue at prescribed times. These times may berepresented as t₀ +t, (t₀ +t₁, t₀ +t₂, etc.) reflecting the position ofservo bits "smm" with respect to a fixed standard (e.g., Start RadiusSR, see FIG. 1; as known in the art--to indicating radius SR). Accordingto a feature hereof, misregistration of the transducer gaps may bedetected in terms of time-variations from these "reference times" sensedfrom associated output servo signals (e.g., t₀ +t₁ +t_(v) ; t₀ +t₂+t_(v), etc.).

Such a misregistration is very schematically indicated in FIG. 3A wheretransducer gaps g, g' of head assembly TR are misregistered "leftward"of track T-1 (assuming the gaps pass in the direction indicated by thearrow), or displaced "radially outward" of disk D. Track T-1 will beunderstood to include work-bits aligned to be sensed by gap "g", plustransverse servo-bits "smm" aligned to be sensed by gap g'. Such"leftward misregistration" of head TR will be noted as causing gap g' tointersect (a portion of) servo-bits "smm" somewhat "late" and produce anassociated servo output pulse "later" in time (delay Δ=+t_(v)) than thecase where gaps g, g' are centered exactly along track T-1. Conversely,"rightward" misregistration of gaps g, g' would cause the g' output tooccur "sooner" (delay Δ=-t_(v)). Comparison of either such misregisteredservo output pulse with a train of "reference pulses" representing the"centered head" (or registration) condition, will yield a timedifference value ±Δt which may be used to determine the direction, anddegree, of misregistration--as workers can visualize (see FIG. 2description details).

Workers will recognize that such an "orthogonal" magnetic "overwrite" ofservo-bits (upon work-bits will, if the bits are substantiallyorthogonal), render each bit type uniquely responsive to only one of thedi-gaps and magnetically isolated from the other bit type. That is, asworkers in the art well know, a bit disposed transverse to a transducergap is relatively "de-coupled", magnetically therefrom andnon-responsive thereto. Also, adjacent orthogonal magnetic domains haverelatively little significant magnetic interaction with one another.

For instance, consider bit "smm", in FIG. 3A: the net effect of itsmagnetic domains will be to produce a relatively strong magnetic outputsignal (sharp magnetic transition when gap g' passes over "smm" inoperative magnetic relation or "magnetic congruence", therewith), whilebeing relatively isolated from (no significant magnetic interactionwith) "orthogonal" magnetic domains "d" and essentially, "ignored" bypassing orthogonal transducer gap "g" (no output induced therefrom).Conversely, gap "g" will produce no significant servo output signal fromthe passage of bit "smm"; but only from passage of "d".

Restating this from a somewhat different viewpoint, the instant aservo-bit "smm" passes in perfect registration under gap g', it will(momentarily) essentially "fill" the gap 100% with magnetic flux fromits magnetic domains (--here aligned in an East-West direction, andconstituting an "E-W flux").

The transverse magnetic domains "d", will (especially if they are widelyspaced so that only a few intersect bit "smm") present a transverse("N-S") flux--a flux that fills only a relatively small portion ofelongate gap g' (here the "servo-gap"). And their transverse("North-South") magnetic alignment will have little or no interactionwith the "smm" (E-W aligned) domains. As a consequence, bits "smm" willbe relatively "isolated", magnetically, from bits "d" along the track;also bits "smm" can generate a servo output, upon passage of gap g' thatis relatively independent of bits "d" and of associated "work gap" "g".

Workers may wonder whether misregistration of the transducer head canseriously degrade read-out from a "selected" track and/or introduce"error signals" from bits along the adjacent track--since the tracks areabutted together. For instance, in the embodiment of FIGS. 3, 3A,misregistration of gap g' to the right (or radially-inward from T-1)would overlap track T-2. This would likely a attenuate the "T-1 servooutput" (from servo-bits "smm" along track T-1,--to the extent g' is outof "congruence" with gap g') as well as erroneously induce a pick-up ing' from work-bits "d" in adjacent track T-2 (--to the extent theyregister with g').

But, workers are quite familiar with known techniques adaptable todistinguish such servo-bits from such work-bits. For instance, thework-bits may be recorded at a particular high frequency, and/or bespaced closer together than servo-bits. In such a case the work-bitread-out can readily be filtered-out by conventional frequencydiscrimination techniques--the servo-bits along a given track beingreferenced to a prescribed known "reference spacing" (timing interval)and thus identified from the raw output pulses. (e.g., the mentioned"reference pulses" from the index hole could be used for this). Also,(as above-mentioned) MIN-amplitude discrimination can set a limit onwhich signals are accepted after such misregistration.

By contrast it will be appreciated that servo di-bits (chevrons) likethose indicated in FIGS. 1 and 2 are, for some purposes, more desirableand efficient than single servo-bits--for instance, they are"self-referencing", in the sense of needing no external "reference pulsetrain" for such purposes. For instance, a control servo pulse train foreach track may be stored in "memory" and keyed to the track number andto a prescribed disk rpm. Those may be used as the "reference pulses" tobe compared with the raw servo pulses, with any associated errors(lead-time or lag-time) computed with reference thereto.

The significance of the aforementioned dual-gap transducer array and theassociated "adjacent orthogonal" alignment of data/servo bits andrecording tracks will be evident. Such a "di-bit/di-gap" type"track-on-data" technique is contemplated in the described, and otherrelated recording/servo positioning systems. Such systems will be betterunderstood by consideration of the explanation indicated in FIG. 3B anddescribed below as follows.

Fig. 3b:

an arrangement like that of FIG. 3, 3A is very schematically shown inFIG. 3B wherein similar data magnetic bits "dd" are obliquely alignedalong the indicated track segment, with transverse singular "servo" bits"ch" similarly arranged at selected regular intervals along the track,but aligned orthogonal to data bits "dd", and laid across several ofthem. Here, (as above) it will be understood that a di-gap transducerarrangement will present two gaps, each to be uniquely parallel andaligned with bits from only one set (see gap g_(a) for work-bits "dd",and gap g_(b) for servo-bits "ch" in FIG. 3B). The "data output pulses"(when bits "dd" are sensed by g_(a)) are represented in idealizedfashion by pulse train S-A in prescribed chronological/spatial relationwith the depiction of work-bits "dd". A related train of "centered"servo output pulses" S-B is similarly chronological and spatiallyreferenced to servo-bits "ch", (the output from gap g_(b)). It will beunderstood that, when exactly "centered", gap g_(b) detects successiveservo-bits "ch" to produce the pulses S-B, peaking at the indicatedtimes (t_(o), t_(o) ', etc.) separated by prescribed normal clockinterval, or frequency f_(o) (reflecting the corresponding spacing andrelative transition speed of the servo-bits relative to gap g_(b)). Butif the head is "shifted left", off-center, then a time-shifted pulsetrain S-B' will be generated; whereas a "shifted-right" will generatepulse train S-B".

In light of the above explanation, workers will recognize thatmisregistration of the di-gap transducer assembly (particularly gapg_(b) therein) to the left of the indicated translation direction (seedirectional arrow on the track FIG. 3B) will produce a train of clockpulses S-B' which will lead the normal "registration-train" S-B by aprescribed "error time" (-Δt_(i)), the magnitude of which isproportional to the degree of (leftward) misregistration (note negativevalue of Δt_(i) indicating "leftward error", i.e., a "leading relation"to time t_(o)); whereas a rightward misregistration is indicated bypulse train S-B", namely servo pulses which lag the normal pulse timet_(o) by prescribed positive error-time (+Δt_(ii)). As before, one may"center" the transducer simply by comparing the raw servo output [S-B'or S-B"] with a "reference pulse" train [S-B; e.g., stored in memory andissue in prescribed time-relation with the "index pulse"], and using thetime-difference (±Δt) to control a conventional servo system (viafeedback control, etc.).

It will be appreciated that a "di-bit/di-gap; track-on-data" arrangementand technique as described in the indicated and other relatedrecording-servo positioning systems may be advantageously employed. Sucharrangements will be seen as especially advantageous in conjunction withan array of "abutting" of data tracks wherein the work-bits andservo-bits are in orthogonal relation between adjacent tracks and amongthemselves along a given track.

The above description has been concerned with the manner in which adi-gap head assembly may be precisely positioned among a plurality ofselectable abutting track positions using di-bit indicia along thetracks. As pointed out previously, in a conventional type of magneticdisk system to which the present invention may be applied one couldemploy a plurality of such head assemblies, each arranged to operatewith a different sub-set of such tracks--whether provided on a disk orother record medium. It will be understood that the positioning of eachsuch head assembly relative to its data zone may be provided in the samemanner as described herein. It will also be understood that, in order tomore efficiently employ space on the disk, it is possible to intermingle"work-bits" with orthogonal "position bits" along a track.

However, it will ordinarily be preferred to reserve certainpre-designated "track sectors" for the servo-bits as a means ofmaintaining the integrity of the servo-indicia inviolate. For instance,a certain amount of "bit-creep" is unavoidable in the course of dozensof cycles of erasing and rewriting work-bits--with the result that,unless an appropriate servo-sector is so reserved, work-bits can shiftin position along a track until they eventually creep into the area ofthe servo-bits, and become confused therewith. (Unless, of course, theentire track is erased and rewritten, including all servo-bits, witheach data update, however minor).

It will also be understood that di-gap heads according to the inventionwill be applied as generally known in the art. One suitable, preferredtype of application is suggested by control system 200 veryschematically shown in FIG. 5. System 200 is generally structured andadapted to control the output from such a pair of associated transducers(H_(a), H_(b) mounted on a common head mount HM) and implement theassociated head repositioning and centering. That is, system 200includes a switch means 211 adapted to route transducer output (alsoinput) according to which head-gap is detecting work-bits and which isdetecting servo-bits. For instance, according to one preferred dataformat (mentioned above) servo-bits would align with one gap along alleven-numbered tracks (e.g., as shown here for gap H_(a) --note arrowindicating passage of track bits) and with the other head (e.g., gapH_(b), here) for all odd-numbered tracks--with work-bits having thereverse format (e.g., here aligned with gap H_(b) for this even-numberedtrack).

Accordingly, switch means 211 must be alternated between tracks toimplement such a format--e.g., being controlled by an ODD-EVEN counter205 and associated control 207 responsive to a track address register201 as shown here. The switch means will include an actuator unit asknown in the art (here, only suggested in phantom at 212) and compriseany suitable known mechanism for switching the paired-gap outputsbetween work-data and servo-data lines.

Such switching of the gap outputs will also involve alternatelyconnecting each gap to a servo feedback loop, implemented as known inthe art and, here suggested by comparator (COMP) stage 209 together withposition control (circuitry) stage 203, controlling actuator assembly213 to position (center) the head mount HM over a given track, as iswell known in the art.

"COMP" stage 209 preferably is coupled to receive the "work-data" outputalso, as shown, and includes means for comparing (the synchronism ofthe) related orthogonal servo-bits (e.g., like marks "sm" in FIGS. 1,1A, and generating an output (preferably amplified and digitized),representing the degree of head misregistration according to howsynchronous the half-bits are (e.g., as in explanation of FIG. 2)."COMP" stage 209 is also adapted to properly enable, or "gate" thiscomparison output so it issues only when the desired servo-indicia aredetected by the paired gaps. One means of doing this is to "gate fromthe servo pattern itself"--such as by detecting an appropriate "preamblepattern" on the track, or doing so electronically "on the di-bit patternitself" [e.g., storing di-gap output briefly in a special overflowregister and gating-out the contents for servo-use only after detectionof this special di-bit indicia]. Workers will contemplate specific knownmeans for effecting this, or for alternative gating. Another gatingoption is illustrated in phantom at "S-S DET" (a "servo-signaldetection") stage 208, which, as known in the art, is adapted torecognize this servo "di-bit" (or related indicia) and, responsively,issue an "enable" signal to so cause stage 209 to issue the mentionedservo signals, applied to "Position Control" stage 203.

Workers will appreciate how aptly such an output processing systemcooperates with such "di-bit/di-gap" arrangements according to theinvention. In particular, the combined "di-bit comparing" and"output-switching" means co-act with the rest of a conventional servosystem to effect efficient head-registration without the shortcomings ofconventional techniques--e.g., with susceptibility to erroneousamplitude-fluctuation (e.g., due to noise, velocity shift, etc.), or toerroneous or complex velocity variations (e.g., in turntable or in delayor clock mechanisms), etc. Details of this and like implementation arewell known to workers (e.g., see similar arrangements in U.S. Pat. No.4,007,493 to Behr, et al.; U.S. Pat. No. 3,903,545 to Beecroft, et al.;U.S. Pat. No. 3,185,972 to Sippel and U.S. Pat. No. 3,686,649 to Behr,and in other patents cited above).

It will be understood that the preferred embodiments described hereinare only exemplary, and that the invention is capable of manymodifications and variations in construction, arrangement and usewithout departing from the spirit of the invention. For example,although it has been assumed that the normal "centered" position along atrack would be indicated by indicia on the track--while this ispreferable, giving real time/space control data, it is not necessary,since such indicia may be stored in memory and fed to compare means withactual position read-out, as workers can visualize.

Also, the same sort of "paired-gap" transducer array may be differentlyrendered; for instance, being implemented with each gap "subdivided" asindicated for the exemplary gap "g-sw" in FIG. 6. Gap "g-sw" isillustrated as straddling a portion of a typical data track T" (seearrow) with a pair of illustrative skewed magnetic transitions "sm"(--to be transduced, as above, by gap "g-sw"--), plus a few orthogonaltransitions "d" (to be transduced by the companion gap to "g-sw", notshown). Gap "g-sw" will be understood as subdivided into an appropriatenumber of sub-gaps (six indicated here, defined, respectively, betweentransducer parts H-1a/H-1b, H-2a/H-2b, etc.). Now, as gap "g-sw" isshifted relative to a track T", it will be apparent that some of thesesub-gaps will be moved out of registry with the passing bits.Accordingly, each sub-gap may be connected to a separate output circuit(where economically feasible) which is adapted to detect "bit-registry"as a pre-condition to applying its detected data output signals to autilization stage, in company with all other such "registry-gated"outputs. The summed "sub-outputs" will constitute the net output signal.

Further modifications of the invention are also possible. For example,the means and methods disclosed herein are also applicable to tapesystems and the like, as well as to drums etc. Also, the presentinvention is applicable for providing the positioning required in otherforms of recording and/or reproducing systems, such as those in whichdata is recorded and reproduced optically.

The above examples of possible variations of the present invention aremerely illustrative. Accordingly, the present invention is to beconsidered as including all possible modifications and variations comingwithin the scope of the invention as defined by the appended claims.

What is claimed is:
 1. An improved magnetic recording subsystemincluding an improved magnetic head arrangement adapted to record and todetect first and second associated recording bits along prescribed trackportions of a magnetic disk record medium, these bits being skewedrelative to elongate axis of the track and disposed orthogonal to oneanother, this subsystem being arranged to apply the output detected fromsaid first bits to a first data utilization means and to apply theoutput detected from said second bits to a second data utilizationmeans, said subsystem comprising:di-gap magnetic transducer meansincluding at least one pair of skewed relatively transverse transducergaps, each gap being positioned and adapted to align along a differentrespective disk track being spaced apart a prescribed interval, eachhaving an associated output connection; and output switching meansadapted to shift the respective outputs from each said pair of gapsperiodically between said first and second utilization means, each gapin such a pair thus being devoted to transducing only one of said bittypes along a given track, with the other gap devoted to transducing tothe other bit type, these respective bits and gaps thus being alignedparallel along any given track, the bit types being thus selectivelyinterchangeable as to associated utilization means from track to trackas desired and encoded, this relationship being followed by suchswitching means.
 2. The combination recited in claim 1, wherein saidsecond utilization means comprises an electronic servo comparison stageadapted to determine the synchronousness, or time difference, betweenoccurrence of selected first bit and second bit outputs from as detectedby said gaps, given a prescribed gating signal whereupon associatedservo control signals reflecting any such time difference areresponsively issued, being applied to a prescribed servo control meansadapted to control the position of said head relative to the subjecttrack;said bit outputs thus constituting respective "servo" and "work"outputs; each of said gaps thus being interchangeable betweentransduction of servo-bits and work-bits accordingly as their outputsare connected to utilization means by such switching means.
 3. Thecombination as recited in claim 2, wherein said subsystem is adapted totransduce said bits along the concentric tracks of magnetic diskrecording media;wherein the gaps in each said pair are positionallyfixed relative to one another so as to facilitate theposition-referencing of said selected bits as servo-registration bits,and whereby the associated output signals provided by said gap outputsin turn facilitate feedback servo control of head registration, saidcomparison means output thus functioning as "error servo signals"adapted to provide improved centering of said head relative to a subjecttrack.
 4. The combination as recited in claim 3, wherein said first andsecond bits constitute work-bits and servo-bits, respectively, selectedones of said bits being positionally arranged to "match" a prescribedtransducer gap pair so that when these gaps pass in "magneticcongruence" therewith, the transducer output reflecting their detectionmay be used at said comparison means to indicate, and to control, therelative centering of said head and said gaps with respect to thesubject disk track.
 5. The combination as recited in claim 3, whereinsaid bits are shifted 90° in skewed alignment from track to track, andwherein said switching means is adapted to alternately switch therespective gap outputs as said head moves from track to track.
 6. Thecombination as recited in claim 3, wherein said comparison stage isadapted to generate positive and negative centering signals according tothe extent of non-coincidence between prescribed first and secondoutputs from said gap pair, with a "null" indicating perfect headcentering, but with positive and negative time variances, respectively,indicating the extent of leftward and rightward misregistration of saidhead along a subject track.
 7. The combination as recited in claim 3,wherein said first and second bits exhibit two different binary modes ofbit-features along a given track, such as may be detected anddistinguished by said gaps and said associated utilization means.
 8. Thecombination as recited in claim 7, wherein said first bits are skewed inparallel at a prescribed angle with respect to each track axis andspaced a prescribed first distance; and wherein said second bits aresimilarly aligned but distinguished therefrom by relatively lessseparation at a second uniform distance.
 9. The combination as recitedin claim 2, wherein said comparison stage is adapted to detect lateralshifts in head position relative to a subject track and so control headcentering as a function of the existence and extent of varying timeintervals between the transducer outputs corresponding to detection ofsaid selected bit pair, these outputs being processed and decoded bysaid comparison means and used to generate related servo control signalsfor head centering.
 10. The combination as recited in claim 2, whereinsaid first bits comprise work-bits and said second bits comprisetransverse servo-bits.
 11. The combination as recited in claim 10,wherein at least some of said servo-bits are recorded in one or moreservo sectors along each subject track, these sectors being maintainedinviolate and free of other recording marks, thus facilitating servocontrol and head centering without need for a separate servo recordingtrack or associated separate servo transducer means, and without needfor amplitude modulated or frequency modulated detection of said bits.12. The combination as recited in claim 11, wherein each said sectoralso includes "distinguishing preamble bits" and identification bitsidentifying the associated track record.
 13. The combination as recitedin claim 11, wherein said disk tracks are disposed in abuttingrelationship, wherein the alignment of said work-bits and said servobits is interchanged from track to track in a "herringbone" bit patternand wherein said servo sectors are each characterized by flanking tracksegments which are maintained "blank" and free of recording andassociated interference.
 14. The combination as recited in claim 13,wherein the outputs from said paired gaps is switched, conjunctively, bysaid switching means between said utilization means from track to track.15. The combination as recited in claim 2, wherein said comparison stageis adapted to receive a prescribed "di-bit" output representing saidselected first and second bits at a prescribed "registration time", andthis time being indicated and enabled by detection of an associated bitpattern, this stage being adapted to responsively issue centeringsignals to a servo control means and so control the positive andnegative repositioning of said head in the course of centering it. 16.The combination as recited in claim 1, wherein at least some of saidsecond bits are overwritten diagonally across respective ones of saidfirst bits, along at least some of the recording track portions.
 17. Thecombination as recited in claim 12, wherein said first and second bitsare disposed about +45° and -45° respectively with respect to each trackaxis, and are adapted to span the track width.
 18. An improved methodfor centering a magnetic recording head along prescribed recordingtracks and improving data readout to utilization means simply accordingto detection of prescribed data bits therealong, said methodcomprising:providing a di-gap magnetic transducer with at least one pairof skewed orthogonal transducer gaps and associated output terminals;providing first and second utilization means and related switching meansfor switching output signals from said terminals alternately betweensaid utilization means according to which track said head is operatingon; recording along said tracks by impressing an array of first magneticbits with one of said gaps so as to be aligned parallel with the gap andoblique with respect to the subject track axis; and also impressing anarray of second magnetic bits likewise aligned parallel with said othergap and oblique to the track axis but tilted at 90° with respect to saidfirst bits; at least one of said first bits and one of said second bitsbeing recorded at a prescribed locus along each track to constitute a"registration di-bit" in "magnetic congruence" with said related gapsand adapted to automatically indicate the degree of head centering uponcomparison of the associated di-gap outputs.
 19. The methods as recitedin claim 18, wherein improved head centering is effected by thefollowing further steps:effecting coarse positioning of said head bymoving the head to the selected track as indicated by servo controlmeans; initiating a "head centering cycle" by monitoring the output fromsaid gap pair until at least one prescribed locus is detected; thenconjunctively detecting the passage of at least one set of said"registration di-bits" and comparing the associated outputs thereof forascertainment of a positive or negative time-variance from perfect"magnetic congruence" between said di-gap and said associated di-bit asa measure of left or right head misregistration along the subject track;and issuing a responsive centering-reposition control signal to saidservo means for repositioning and better centering of said head; thenre-initiating said head centering cycle unless head centering is withinprescribed limits, repeating this cycle until the prescribed limits aresatisfied; then enabling said data read-out to said utilization means.20. The combination as recited in claim 19, wherein during said headcentering cycle said limits are defined by the maximum time discrepancypermissible, as correlated with a prescribed maximum acceptable,positive and negative, head mis-centering; said head centering cyclebeing continually and gradually invoked, even within said limits, tocontinuously and gradually improve the centering of said head along agiven track.
 21. The method as recited in claim 18 and applied to diskmedia, wherein at least one reference track is first prerecorded on thedisk and wherein other recording tracks are position-referenced by being"slaved" to the circumferential position of said prerecorded track usingsaid "di-bit/di-gap" technique to assure concentricity therewith. 22.The method as recited in claim 18, wherein at least some of said secondbits are "written over" at least some of said first bits for improveddata compression and minimal "cross-talk" along a given track withoutsignificant magnetic interference between adjacent bits or with anon-detecting one of the associated gap pair.
 23. The combination asrecited in claim 22, wherein said second bits comprise servo-bitswritten over work-bits comprising said first bits.
 24. The combinationas recited in claim 23, wherein a special work-bit is recorded and usedin combination with a special servo-bit to provide saidregistration-determining "di-bits".
 25. An improved system forpositioning a magnetic transducer head radially relative to amagnetically active surface of a rotating disk and for recording anddetecting data thereon, this system comprising:a first data recordingalong each disk track; a second data recording along each said track,said first recording comprising a plurality of like magnetic transitionsextending across each track at a prescribed first angle to the directionof motion of said medium; said second recording comprising a pluralityof like magnetic transitions aligned relatively transverse to said firsttransitions along any given track; these transitions being each shifted90° in alignment from track to track across the disk; said transducerhead comprising at least one pair of transducer gaps separated aprescribed distance, each being oriented to align with a respective oneof said first and second recordings along any given track and includinga respective output means; the gaps thus being functionallyinterchangeable with one another so as to be switched in function as thehead moves from track to track; first utilization means; secondutilization means including a compare circuit arranged to be connectedbetween said first and second gap outputs and adapted to provide anoutput centering signal reflecting output mismatch corresponding to theamount of deviation of the head from a predetermined "centered"alignment along a track; servo means connected to said circuit andadapted to reposition the head responsive to said centering signaltoward said predetermined alignment; and switching means adapted toswitch said gap outputs interchangeably between said utilization means.26. The combination as recited in claim 25, wherein said first andsecond recordings are arranged along each track to provide, when sensedby said paired gaps, a specified coincidence of reflecting registrationof said head at said alignment as represented by selected first andsecond recording bits spaced to assume "magnetic congruence" with thepassing gap pair when said head is perfectly centered.
 27. Thecombination as recited in claim 25, wherein said first recordingscomprise an array of control recordings along each track and adapted toindicate track position; wherein said second recordings comprise workdata, both arrays being aligned along concentric tracks but aligned inrespective first and second directions oblique to the track axis andorthogonal to one another along any given track; each paired gap beingadapted to be operatively, transducingly associated with a respectiveone of said recordings to provide related output pulses, selected onesof said first and second recordings being arranged as di-bits along eachtrack to provide, when detected by the associated paired gaps, aprescribed "centering" time relation between associated outputs, saidgaps being paired in prescribed relation and orthogonal to one another,with the gaps thus being adapted and positionally related to providethis time-relation between di-bit outputs indicating degree ofhead-centering; head-positioning servo means and output switching meansadapted to selectively connect said paired-gap outputs, alternatively,to respective data utilization means and said servo means; said comparecircuit being connected to said servo means and adapted to provide asignal reflecting the deviation of the head from perfect center along atrack, said servo means being adapted, responsive to said signal, totranslate the head radially toward track-center.
 28. The system of claim27, wherein said recordings are, each, aligned orthogonally from trackto track and wherein said compare circuit further comprises means foramplifying and digitizing the outputs from said paired transducing gaps.